The present invention relates to an arc discharge abnormality detecting system and, more particularly, to the system for detecting the abnormality of arc discharge in generating source plasma by arc discharge using a plurality of filaments as cathodes in an ion source used in a neutron beam injector.
Generally, in the ion source used in the neutral beam injector, the source plasma is generated by a direct current arc discharge of a large current (.about.1000 A) because of the necessity for producing an ion beam of a high density and a large sectional area. A plurality of filaments are used generally as an arc discharging cathode for a large electric current. In a normal arc discharge in this ion source, the arc discharge current flows substantially uniformly over the entire surface of these filaments. In an abnormality of the arc discharge, however, the arc discharge current flowing in the source is concentrated at a portion of the surface of the filament, causing local overheating and melting at that portion of the filament. This results in the disadvantage of considerably decreasing the life of the filament.
FIG. 1 shows schematically an example of a conventional arc discharge abnormality detecting system, in which an ion source 1 is hollow and includes filaments 2 and 3 therewithin. The example shown in FIG. 1 employs the two filaments. The arc discharge occurs between the inner faces of the walls forming the ion source 1 and the filaments 2, 3 and source plasma is generated over the entire enclosed volume. One terminal of the arc power supply 4 is connected to the wall of the ion source 1 through an arc discharge cut-off switch 9 and a current detector 8 for measuring the entire arc discharge current while the other terminal thereof is connected to one end of each of the filaments 2, 3 through respective resistances 7 for averaging the arc current flowing through the filaments. Designated by 5 and 6 are filament power supplies. The circuit, illustrated in this example, is designed for judging the abnormality of the arc discharge and opening the switch 9 when the indication of the current detector 8 exceeds a predetermined value. This utilizes the phenomenon that the resistance of the arc discharge is lower during abnormal arc discharge than is normal, thereby increasing the arc discharge current. In the abnormal arc discharge, however, there is a case where a large arc discharge current flows in one filament while substantially no arc discharge current flows to the other filament and, therefore, there is substantially no change in the entire arc discharge current measured by the current detector 8. The conventional detecting system of the example shown in FIG. 1 has a disadvantage that the abnormal arc discharge like this is not detectable thereby.
FIG. 2 shows schematically another example of a conventional arc discharge abnormality detecting system, in which reference numerals 1 to 7 and 9 denote the parts corresponding to those of FIG. 1, 11 denotes a probe for determining ion saturation current or floating potential, and 10 denotes a logic circuit for judging arc discharge abnormality from a signal from the probe 11. In this example, the logic circuit 10 judges whether the arc discharge is abnormal or not by means of the signal from the probe 11 and opens the switch 9. This utilizes the phenomenon that the ion saturation current or floating potential changes when the arc discharge is abnormal. However, since these changes are not uniform at all the times, the conventional detecting system of the example shown in FIG. 2 has a disadvantage that it is difficult to form the logic circuit 10 so as to judge the normality or abnormality of arc discharge without fail.